The Advanced Television Systems Committee (ATSC) published a Digital Television Standard in 1995 as Document A/53, hereinafter referred to simply as “A/53” for sake of brevity. Annex D of A/53 titled “RF/Transmission Systems Characteristics” is particularly incorporated by reference into this specification. So is Section 5.6.3 titled “Specification of private data services” from Annex C of A/53.
In 2007 Samsung Electronics Co., Ltd. proposed introducing turbo-coded ancillary data into adaptation fields of the 187-byte MPEG-2-compatible data packets included in the 207-byte data segments of the 8VSB DTV broadcast signals used in the United States. This scheme called “AVSB” is championed because the packet decoders in legacy DTV receivers can readily disregard the turbo-coded ancillary datastream, providing a form of backward compatibility in which those legacy DTV receivers can still receive a principal datastream transmitted in the data fields of the 187-byte MPEG-2-compatible data packets. Variants of AVSB should be possible in which the turbo-coded ancillary datastream occupies 184-byte payload fields of the 187-byte MPEG-2-compatible data packets, it is here noted. Other variants of AVSB should be possible in which the turbo-coded ancillary datastream occupies complete 207-byte data segments, it is also here noted.
In its turbo coding AVSB uses an outer encoder similar to that used in the Universal Mobile Telecommunications System (UMTS) specification of the European Telecommunications Standards Institute, as standardized by the Third-Generation Partnership Project (3GPP). The original data and the outer encoding parity bits are bit interleaved. Then, the ⅔ trellis coding specified in A/53 provides the inner coding of the turbo coding.
The 8-level symbol mapping specified in A/53 maps each group of Z2, Z1 and Z0 bits into a respective eight-level VSB symbol in accordance with simple binary coding. This results in the Z2 and Z2 bits of original data both changing value between the 011 and 100 levels. This makes a double-bit error likely when noise causes an adjacent-bin error during data slicing in this region of the symbol map. Only Reed-Solomon coding with 8-bit bytes is concatenated after the ⅔ trellis coding in ordinary 8VSB transmissions as specified by A/53, so the double-bit errors being within single bytes affect overall coding being found correct no more than single-bit errors within single bytes. However, when further convolutional coding is introduced after the ⅔ trellis coding, the double-bit errors are more disruptive than single-bit errors.
Digital transmission system using multi-level symbols generated by Gray coding are known. An adjacent-bin error will cause only a single-bit error in an 8-level symbol using Gray code symbol mapping, rather than a double-bit or triple-bit error. However, symbol mapping using Gray code over all eight modulation levels is incompatible with ⅔ trellis coding, especially when bytes of AVSB coding are interleaved with bytes of ordinary 8VSB coding. So, initially, the inventor was unable to discern how to utilize effectively the general idea of avoiding an adjacent-bin error during data slicing generating double-bit errors in a special type of turbo coding designed for digital television broadcasting.
After further consideration, the inventor was able to figure out how to avoid an adjacent-bin error during data slicing generating double-bit errors in nibbles each composed of a Z-sub-2 bit and a Z-sub-1 bit. (In this specification and its accompanying drawing, the word “nibble” refers to a pair of bits. This is a departure from conventional usage in digital electronics, in which conventional usage the words “nibble” and “nybble” refer to a group of four bits that are half of an 8-bit byte. In this specification and the claims that follow the term “anti-Gray coding” refers to coding of groups of bits that when followed by Gray coding results in recovery of the original groups of bits. This is different from another definition of “anti-Gray coding” used in digital electronics wherein successive levels of a plural-level code exhibit maximum Hamming distances between adjoining levels.) Each 2-bit nibble composed of a Z-sub-2 bit and a Z-sub-1 bit could be anti-Gray coded before ⅔ trellis coding in the DTV transmitter. Then, subsequent to ⅔ trellis decoding in the DTV receiver, each 2-bit nibble could be Gray coded to counter the effects of the anti-Gray coding. Surprisingly, this procedure converts the symbol mapping into modulation levels to Gray coding insofar as the two more significant bits of the 3-bit symbols are concerned. Another surprising result of this procedure is that extends the effects of the ⅔ trellis decoding from just the Z-sub-1 bits to the Z-sub-2 bits as well. After this insight into how to avoid an adjacent-bin error during data slicing generating double-bit errors in nibbles each composed of a Z-sub-2 bit and a Z-sub-1 bit, there remained further problems of designing DTV transmitter and DTV receiver configurations to exploit the insight.